(1) Field of the Invention
The present invention relates to a process for controlling the degree of dispersion of a metal/carrier solid catalyst. This process can be generally applied to various processes for the production of catalysts of this type.
(2) Description of the Related Art
As the preparation process for dispersing a catalyst, the following processes are known; [Catalyst Lectures, volume 5, Catalyst Design, page 39 (1985), compiled by Catalyst Society of Japan and published by Kodansha, and ibid, Special Edition, Catalyst Experiment Handbook, page 15 (1986)].
(1) Impregnation Process
This process is well-known as a simple means of preparing a carrier-supported catalyst. According to this process, a solid acting as the carrier is immersed in a solution containing a compound of a catalyst active component, or a solution of the catalyst active component is dropped into the carrier, and the mixture is stirred and optionally heated. After a certain time, excess solvent is removed, the residue is dried and fired at a high temperature, and an activation required for each catalyst is carried out. According to the means for impregnating the carrier with a starting material of the catalyst active component, this process is divided into an adsorption method, a pore-filling method, an incipient wetness method, an evaporation-to-dryness method and a spray method.
(a) Adsorption method PA1 According to this method, the amount of a catalyst active component, such as a metal ion, to be adsorbed in a carrier, is predetermined, and an amount, smaller than the saturation absorption amount, of the catalyst active component is completely adsorbed. Note, the method in which a carrier is immersed in a solution containing the active component in an amount exceeding the saturation absorption amount, and the excess portion is removed by filtration, is called an equilibrium adsorption method. PA1 (b) Pore-filling method PA1 As seen from the name thereof, according to this method the pore volume of a carrier is measured, a solution of an active component is added to the carrier in a volume equal to the measured pore volume, and all of the active component is sucked into the carrier. PA1 (c) Incipient wetness method PA1 According to this method the impregnation is performed while measuring the pore volume of a carrier. More specifically, while a carrier is being stirred, a solution of an active component is gradually added to the carrier by a buret or the like, and the addition is continued until a state is reached wherein the surface of the carrier is uniformly wetted and no excess solution is present. The amount supported of the active component is adjusted by changing the concentration of the active component in the solution. PA1 (d) Evaporation-to-dryness method PA1 A carrier is immersed in a solution of an active component and the mixture is heated with stirring on a hot water bath or the like, to evaporate the solvent (water or the like) and dry and fix an intermediate of the active component to the carrier. Where an increase of the amount supported of the active component is desired, or the compatibility between the carrier and the catalyst component is low, this method is adopted. The method is defective, however, in that the active component is not uniformly dispersed. PA1 (e) Spray method PA1 A carrier is charged in an evaporator and stirring is conducted while removing air. The carrier is always kept in a dry state and a solution of an active component is sprayed onto the carrier to effect impregnation.
(2) Ion Exchange Process
A zeolite, zirconium phosphate or an oxide having a surface acidity is used as a carrier, and the carrier is immersed in a solution containing an active component as a cation to effect an ion exchange and support the active component on inner walls of pores of the carrier. Post treatments are conducted according to the procedures described above with respect to the impregnation process.
(3) Co-precipitation Process
A precipitant is added to a mixed solution containing a carrier component and a catalyst active component to simultaneously form a precipitate of both components. Namely, solutions of both components are separately dropped into a solution of the precipitant or a mixed solution is dropped into the solution of the precipitant, whereby a precipitate is formed. The formation of the precipitate is influenced by various factors such as the starting materials, the kind of solvent, the solvent concentration, the amount of solution, the kind of precipitant, the pH value, the solution-precipitant mixing method, the stirring speed, and the temperature. As the treatment after the precipitation, an aging, ion exchange and washing are carried out according to need. Post treatments after the precipitation and drying are carried out according to the procedures described above with respect to the impregnation process.
(4) Deposition Process
A carrier is immersed in a solution of a catalyst active component, and a precipitant is added to the mixture with stirring to deposit a precipitate of the active component on the carrier. Often the precipitate of the active component is unevenly deposited, and as the means for avoiding this disadvantage, there is known a uniform precipitation method using urea as the precipitant.
(5) Kneading Process
A precipitate of at least one catalyst active component is preformed, a powder, hydrogel or hydrosol of a starting material of a carrier is added to the precipitate, and the mixture is kneaded by a ball mill or a kneader.
(6) Hydrothermal Synthesis Process
This process is applied to the synthesis of a zeolite or lamellar silicate catalyst. The process is adopted when a starting material of a catalyst active component having a low water solubility is used, and according to this process, a catalyst crystal is synthesized in a high-temperature and high-pressure aqueous solution having a required solubilizing power. In general, a mineralizing agent such as (an alkali) is added, to increase the solubility of the crystal.
(7) Melting Process
At least two catalyst active components are melt-mixed at a high temperature in an electric furnace. An oxide (double accelerating iron catalyst formed from Fe.sub.3 O.sub.4 --Al.sub.2 O.sub.3 --KNO.sub.3 as the starting material) is used, or alternatively, a metal (an Ni-Al alloy for a Raney catalyst) is used. When a metal is used, the melting is carried out in a reducing atmosphere (such as Ar or N.sub.2).
(8) Gas Phase Synthesis Process
This process is characterized in that fine powders of various catalysts are synthesized. The process is divided into (a) a method of evaporation-condensation of a starting material of a catalyst and (b) a method in which a chemical reaction is caused in the gas phase. According to the method (a), a starting material is heated by arc or plasma, and a vapor formed by gasification is rapidly cooled at a large temperature gradient possessed by an arc or plasma flame, to condense the vapor into fine particles. According to the method (b), fine particles of an intended substance are synthesized by thermal decomposition of a vapor of a volatizable catalyst compound or gas phase reaction of the vapor with other gas. A metal chloride, which has a high vapor pressure and a relatively high reactivity, is often used as the starting material of the catalyst in this method.
Each of the processes (1) through (6) is a process (wet process) utilizing a solubility or dispersibility of the active component in a liquid phase at normal temperature, and in each of these processes, a cumbersome operation of removing the solvent after the dispersion must be performed, or a problem of an increase in the costs inevitably arises. Furthermore, the used active component is not completely utilized, and thus a considerable amount of the active component is lost.
Each of the processes (7) and (8) is a dry process and a solvent is not used, and therefore, the above-mentioned problems are overcome. Nevertheless, since the dispersibility at a considerably high temperature in a liquid phase state or gas phase state is utilized, the labor and costs are still high. Furthermore, the applicable catalysts are very restricted.
A more serious problem arises in that the dispersion is not possible to be freely controlled. In each of the processes (1) through (8), a high dispersion can be effectively attained, but it is difficult to control the degree of dispersion.
It is known that, as a catalyst is highly dispersed the activity is generally enhanced, and accordingly, to increase the activity, it is necessary to attain a high degree of dispersion. If the dispersion degree is excessively increased, however, even though a high activity is temporarily attained, the activity is reduced with the lapse of time by sintering or the like, and in the long run, the activity is reduced. Therefore, most preferably the active component is supported at an appropriate degree of dispersion.